1. Field of the Invention
This invention relates to a method of terminating polydiorganosiloxane to eliminate hydroxy groups on silicon in the finished polymer.
2. Background Information
In the process of producing elastomers of polydiorganosiloxane, it becomes necessary to produce the high molecular weight polydiorganosiloxane used as the backbone of the elastomeric composition. The elastomeric composition comprises the polymer, a filler, and a curing system. The exact polymer that is used depends upon the desired physical properties of the final cured elastomeric composition, as well as the curing system chosen. In the instant case, the primary interest is in those polymers which are of a viscosity such that they are commonly referred to as "gums". These polymers are those usually combined with colloidal silica as a reinforcing agent and with organic peroxides as a curing system. The compositions are formed into shape by molding under pressure or by extrusion, then are cured by exposing to an elevated temperature. The cured elastomer has exceptional heat resistance as well as very acceptable physical properties.
A typical polydiorganosiloxane is ordinarily prepared by a batch process as follows: siloxane cyclic monomers, such as octamethylcyclotetrasiloxane, are charged to a polymerization vessel and dried by distillation or by treatment with drying agents. The monomer is then heated as to about 155.degree. C. and sufficient polymerization catalyst, such as potassium hydroxide, is added to give a concentration of about 5 to 50 parts potassium per million parts of monomer. The polymerization proceeds until the polymer formed has the desired viscosity. The polymerization is stopped by the addition of a neutralizing agent such as an equal molar quantity of acidic material, such as phosphoric acid or carbon dioxide. The crude polymer is then stripped of unreacted monomer by distillation under vacuum. The polymers used in producing heat cured elastomers normally have a viscosity of greater than 1000 Pa.s at 25.degree. C.
The above process is modified to give better control over the final viscosity of the polymer by adding end blocking units to the process during the polymerization process to control the polymer molecular weight. Commonly used end blocking materials are short chain siloxanes having R.sub.3 SiO- ends, where R is hydrocarbon radical or hydroxyl radical or mixtures.
When it is desired to make polymers for use with reinforcing colloidal silica, it is often desirable to have as few hydroxyl groups present as possible. Any hydroxyl groups present react upon contact with the colloidal silica to form polymer-filler linkages which act the same as crosslinks, thus creating a higher viscosity, crosslinked mixture which does not flow easily under pressure. The resultant process is known as "creping". The creped material must be broken down by intensive shearing, as on a two roll mill, before it can be further processed. This is an expensive process, which also can effect the physical properties of the resultant cured elastomer. The endblocking materials are ordinarily triorgano endblocked so that the polymer has few hydroxy radicals present. Another method of attempting to prevent some of this problem is the use of "process aids" in the composition. A typical process aid is a low viscosity, hydroxyl endblocked polydiorganosiloxane. The thought is that the low viscosity material will preferentially react with the filler, thus preventing filler reaction with the high viscosity polymer.
In the polymerization process for making high molecular weight polymers, it is thought that hydroxyl groups are formed on a small number of the chain ends. The process of the instant invention is related to replacing these hydroxyl radicals with radicals which do not react with the filler.
Peterson, in U.S. Pat. No. 4,250,290, issued Feb. 10. 1981, teaches a process for the continuous manufacture of diorganopolysiloxane polymers. The process makes use of either water or a triorganosiloxy chainstopper to maintain the efficiency of the static mixer used in the process.
Maass et al, in U.S. Pat. No. 4,439,592, issued Mar. 27, 1984, describe a process for the preparation of triorganosilyl-terminated polydiorganosiloxanes by basic polymerization of cyclic diorganopolysiloxanes and triorganosilyl-terminated siloxanes. Before the polymerization is started, a part of the cyclic diorganosiloxane is distilled off in the presence of the polymerization catalyst to remove as much of the water present in the mixture as possible before the polymerization is started.
Herberg et al., in U.S. Pat. No. 4,551,515, issued Nov. 5, 1985, teaches a process for the continuous polymerization of polydiorganosiloxanes in which cyclopolysiloxane in mixed with one or more chainstopping agents, preheated, catalyzed, polymerized, and neutralized, using a screw extruder. They discuss drying the mixture of tetramer and chainstopping agent by passing through molecular sieves at the beginning of the process to remove water that will otherwise chainstop the polymer with silanol groups. They teach chainstopping agents may be any of those known in the art, such as vinyl chainstopper, trimethylsiloxy chainstopper, or silanol chainstopper.
None of the above processes or methods produces a polymer completely without any hydroxyl groups in the polymer. When only a triorganosilyl endblocked chainstopping agent is used, and the cyclic monomers are dried before use, a polymer can be produced with a minimum amount of hydroxyl group present, but that minimum amount is sufficient to cause a polymer to react with a colloidal silica filler to an extent sufficient to cause creping.
Additional means of producing polydiorganosiloxane having a lower silanol content may be found in U.S. patent application Ser. No. 923,468, filed Oct. 27, 1986, "Neutralization of Catalyst in Polymerization of Polydiorganosiloxane", now U.S. Pat. No. 4,719,276, by Leo Stebleton, owned by the same assignee as the instant application. This invention relates to a method of neutralizing the catalyst used in the alkaline polymerization of polydiorganosiloxane in which the neutralizing agent is a composition of the formula R'.sub.3 SiOC(O)R", where R' is a hydrocarbon radical of from 1 to 6 carbon atoms inclusive and R" is a hydrocarbon radical of from 1 to 8 carbon atoms inclusive. Preferred is dimethylvinylsilylacetate.
Polydiorganosiloxane polymers are commonly produced by polymerization of cyclic materials using an alkaline catalyst. The catalyst is neutralized at the end of the polymerization process, with the resulting salt commonly remaining in the polymer in some small amount. When heated to high temperatures, the catalyst is activated and causes the polymer to be degraded. U.S. Pat. No. 2,739,952, issued Mar. 27, 1956, teaches the use of an organophosphorus compound free of silicon-bonded halogen in conjunction with such a polymer to prevent the polymer degradation upon heating. In U.S. Pat. No. 3,153,007, issued Oct. 13, 1964, it is commented that steam causes undesirable side effects during the devolatilization of gums such as are described in U.S. Pat. No. 2,739,952.